Stabilized amide composition

ABSTRACT

An amide composition containing an unsaturated amide, a sulfur-containing compound, and a weak acid salt. The composition can include an amide in crystalline form or the composition can be in form of aqueous solution. The composition can exhibit improved stability whereby polymerization of the amide is inhibited.

FIELD OF THE INVENTION

[0001] This invention relates to stabilization of an amide such asacrylamide or methacrylamide. An amide such as acrylamide is a usefulsubstance as a starting material for polymers which have manyapplications such as a coagulating agent, a thickener, a petroleumrecovering agent or a paper strength fortifier or a thickening agent forpaper-making industries.

BACKGROUND OF THE INVENTION

[0002] An amide such as acrylamide is industrially produced bysulfuric-acid hydrolysis method which includes a step of heating anitrile such as acrylonitrile with sulfuric acid and water to obtain anamidosulfate, a method of hydrating a nitrile in the presence of acatalyst such as copper metal, a copper oxide or a copper salt to obtaina corresponding amide, a method of hydrating a nitrile using a microbialenzyme (nitrile hydratase) to obtain an amide, a method of producingmethacrylamide from acetocyanohydrin and so on.

[0003] Since an amide is extremely polymerizable, polymerization of anamide can take place in each step of any of the production methodsdescribed above or in preservation or deposition of an amide product ina crystalline state or as an aqueous solution. The polymerization canalso cause problems such as deterioration in quality, decrease in yieldfrom the production process, obstruction to liquid flow in apparatusesor tubing, and decrease in conduction of heat.

[0004] Therefore many kinds of compounds are proposed as apolymerization retarder for an amide. For example, JP-B-39-10109discloses a method to inhibit polymerization of an amide by adding oneor more compounds selected from the group consisting of thiourea,ammonium rhodanide, nitrobenzene, o-tolidine, phenothiazine, and nitrosoR salt to the amide so that they coexist with the amide. FurtherJP-B-2548051 discloses a method for stabilizing an acrylamide solutionusing water-soluble monocarboxylic acid salts having at least two carbonatoms. (The term “JP-B” as used herein means an “examined Japanesepatent publication”.)

[0005] However, the polymerization inhibition effects of the knownmethods are not sufficient, and further improvements have been demanded.

SUMMARY OF THE INVENTION

[0006] The object of the invention is to provide an amide composition,which is more stable than the amide compositions of conventionaltechniques.

[0007] For example, the invention provides the techniques as shownbelow.

[0008] (1) An amide composition comprising an amide which has anunsaturated bond, a sulfur-containing compound and a weak acid salt.

[0009] (2) The amide composition as shown in (1), wherein thesulfur-containing compound is thiourea.

[0010] (3) The amide composition as shown in (1), wherein the amide isan unsaturated aliphatic amide.

[0011] (4) The amide composition as shown in (1), wherein the amide isacrylamide or methacrylamide.

[0012] (5) The amide composition as shown in (1), wherein thesulfur-containing compound is present in an amount in the range of 1 to100 ppm based on the weight of the amide.

[0013] (6) The amide composition as shown in (1), wherein the weak acidsalt is a salt of an organic acid, which has an acid dissociationexponent of 3.5 to 5.5.

[0014] (7) The amide composition as shown in (1), wherein the weak acidsalt is present in an amount in the range of 1 to 3,000 ppm based on theweight of the amide.

[0015] (8) The amide composition as shown in (1), wherein thesulfur-containing compound is thiourea, the weak acid salt is a salt ofan organic acid having an acid dissociation exponent of 3.5 to 5.5, thethiourea is present in an amount in the range of 1 to 100 ppm based onthe weight of the amide and the salt of the organic acid is present inan amount in the range of 1 to 3000 ppm based on the weight of theamide.

[0016] (9) The amide composition as shown in any one of (1) to (8),wherein the amide is in a crystalline state.

[0017] (10) The amide composition as shown in any one of (1) to (8),wherein the composition is an aqueous solution.

DETAILED DESCRIPTION OF THE INVENTION

[0018] Amide

[0019] The amide to be used in the invention has an unsaturated bond. Anunsaturated amide having 3 to 20 carbon atoms is preferable as the amideto be used in this invention. Among them, an unsaturated aliphatic amidesuch as acrylamide, methacrylamide or crotonamide is more preferable.

[0020] The amide to be used in the invention is typically a monomer.However, the invention can be applied to an amide dimer, an amidetrimer, an amide oligomer and so on.

[0021] The amide to be used in the invention can be industriallyproduced by hydrating the corresponding nitrile in the manner ofsulfuric-acid hydration method, acetone cyanohydrin method, coppercatalyst method, or enzyme method. Among them, the copper catalystmethod and enzyme method are most preferable.

[0022] The copper method is a method to produce an amide by directlyhydrating a nitrile in the presence of copper catalyst such as coppermetal, a reduced copper or a Raney copper. Some examples of the methodare disclosed in JP-B-52-33092 or JP-B-55-11657.

[0023] The enzyme method is a method to produce an amide by hydrating anitrile by the action of an enzyme (nitrile hydratase) which is obtainedby a microorganism and capable of hydrating a nitrile to produce acorresponding amide. A microorganism which contains the enzyme, or atreated substance of the enzyme or the microorganism can also be usedinstead of the enzyme itself. Some examples of the enzyme method aredisclosed in JP-A-11-89575. (The term “JP-A” as used herein means a“unexamined published Japanese patent application”.) In the enzymemethod, the amount of impurities such as metal ions from a catalyst issmall, and the amount of by-products is small since the reaction isconducted at room temperature and atmospheric pressure. Therefore, anamide obtained by the enzyme method has a good stability and apolymerization process of the amide is less affected by the impuritiesor the by-products. Thus, the enzyme method is especially preferable.

[0024] The amide thus produced is available either as an aqueoussolution or in a crystalline state by crystallization or drying.

[0025] Sulfur-Containing Compound

[0026] The sulfur-containing compound to be used in the inventionpreferably contains at least one nitrogen atom.

[0027] The sulfur-containing compound to be used in the invention may bethiourea, phenothiazine and so on. Among them, thiourea is mostpreferable. The amount of the sulfur containing compound is preferably 1to 100 ppm, and is more preferably 1 to 30 ppm, based on the weight ofthe amide respectively.

[0028] Weak Acid Salt

[0029] The term “weak acid” as used herein means an acid having an aciddissociation exponent measured in water at 25° C. of 2 or more. The aciddissociation exponent of the weak acid is preferably in the range of 2to 6.5, and more preferably in the range of 3 to 6.

[0030] A salt of an organic acid having an acid dissociation exponentmeasured in water at 25° C. of 3.5 to 5.5 is preferably used as a weakacid salt of the invention. The organic acid may be a saturatedaliphatic monocarboxylic-acid, an unsaturated aliphaticmonocarboxylic-acid, an aliphatic polycarboxylic-acid, an aromaticcarboxylic-acid or the like. Illustrative examples of the saturatedaliphatic monocarboxylic-acid include acetic acid, propionic acid,octanoic acid, valeric acid and the like. Illustrative examples of theunsaturated aliphatic monocarboxylic-acid include acrylic acid, crotonicacid, methacrylic acid and the like. Illustrative examples of thealiphatic polycarboxylic-acid include oxalic acid, adipic acid, succinicacid, maleic acid and the like. Illustrative examples of the aromaticcarboxylic-acid include benzoic acid and the like. Among these organicacids, acrylic acid and methacrylic acid are most preferable.

[0031] The weak acid salt of the invention may be a sodium salt, apotassium salt, an ammonium salt or the like. Among these salts, thesodium salt is most preferable. Sodium acrylate and sodium methacrylateare the most preferable examples of the weak acid salt of the invention.

[0032] The weak acid salt can be added to an amide containing solutionnot only in the form of a salt but also in the form of an acid, providedthe weak acid salt is formed in the solution. The amount of the weakacid salt is preferably in the range of 1 to 3,000 ppm based on theweight of the amide. More preferably it is in the range of 1 to 2,000ppm.

[0033] Other Additives

[0034] Other additives can be used in the invention as needed. Typicalexamples of the additives include alkoxy-phenols such as methoxy-phenoland ethoxy-phenol, nitroso compounds such asp-nitroso-di(phenylhydroxylamine), hydroxylamines, mixtures of metalssuch as manganese and chelate compounds and so on.

[0035] Stabilized Aqueous Solution of Amide

[0036] The stabilized aqueous solution of amide of the invention can bestabilized in the co-existence of sulfur containing compound such asthiourea and weak acid salt with an amide. Other additives can co-existas needed.

[0037] Typical examples of a method to make stabilizers co-exist withthe amide include a method in which the stabilizers are added tostarting materials for producing the amide, a method in which thestabilizers are added in an arbitrary step of the process for producingthe amide, a method in which the stabilizers are added to the aqueoussolution of amide after the process for producing the amide havefinished and so on.

[0038] The aqueous solution of amide of the invention comprises anamide, a sulfur-containing compound and a weak acid salt. The amount orconcentration of the amide in the aqueous solution is not limitedspecifically. However, the concentration is preferably in the range of 5to 80 weight %, and is more preferably in the range of 10 to 60 weight%.

[0039] The amount of the sulfur-containing compound in the aqueoussolution is preferably in the range of 1 to 100 ppm, and is morepreferably in the range of 1 to 30 ppm, based on the weight of the amiderespectively.

[0040] The amount of the weak acid salt in the aqueous solution ispreferably in the range of 1 to 3,000 ppm, and is more preferably in therange of 1 to 2000 ppm, based on the amount of the amide respectively.

[0041] If the amount of sulfur-containing compound and the amount ofweak acid salt are in the range described above, the effect ofstabilizing the aqueous solution of amide during its preservation issufficient. Furthermore, it is industrially beneficial to use thesulfur-containing compound and the weak acid salt in the range describedabove. Because these polymerization retarders are less influential inthe polymerization reaction, in which an amide having an unsaturatedbond is polymerized to produce a corresponding polymer product, thanother polymerization retarders such as copper ions.

[0042] Other additives can be used in the invention as needed. Forexample, an alkoxyphenol such as methoxyphenol can be used. The amountof the alkoxyphenol is preferably in the range of 1 to 100 ppm, morepreferably is in the range of 20 to 50 ppm, based on the amount of theamide respectively.

[0043] Stabilized Crystalline Amide

[0044] The crystalline amide of the invention can be stabilized in theco-existence of sulfur-containing compound such as thiourea and weakacid salt with an amide. Other additives can co-exist as needed. Thesame type of amide, weak acid salt and other additives as describedabove can be used.

[0045] Typical examples of a method to make stabilizers co-exist withthe amide include a method in which the stabilizers are added to thestarting material for producing the amide, a method in which thestabilizers are added in an arbitrary step of the process for producingthe amide and then crystals are formed from the amide, a method in whichthe sulfur-containing compound and the weak acid salt are directly addedto a wet amide crystals which are separated from a slurry in acrystallization step, a method in which thiourea and an weak acid saltare dissolved or dispersed in a medium such as water and sprayed on wetamide crystals and so on.

[0046] Water, solvent and so on are removed from the amide crystals inthe manner of conventional drying under atmospheric pressure or reducedpressure. Then crystalline amide as a product is obtained. Thecrystalline amide of the invention comprises a sulfur-containingcompound and a weak acid salt.

[0047] The amount of the sulfur-containing compound is preferably in therange of 1 to 100 ppm, and is more preferably in the range of 1 to 30ppm, based on the weight of the amide respectively.

[0048] The amount of the weak acid salt is preferably in the range of 1to 3,000 ppm, and is more preferably in the range of 1 to 2,000 ppm,based on the weight of the amide respectively.

[0049] If the amount of sulfur-containing compound and the amount ofweak acid salt are in the ranges described above, the effect ofstabilizing the amide crystals during its preservation is sufficient,and then the crystals are not likely to coagulate.

[0050] Furthermore, it is industrially beneficial to use thesulfur-containing compound and the weak acid salt in the range describedabove. Because these polymerization retarders are less influential inthe polymerization reaction, in which an amide having an unsaturatedbond is polymerized to produce a corresponding polymer product, thanother polymerization retarders such as copper ions.

[0051] Other additives can be used in the amide crystals of theinvention as needed. For example, an alkoxyphenol such as methoxyphenolcan be used. The amount of the alkoxyphenol is preferably in the rangeof 1 to 100 ppm, more preferably in the range of 20 to 50 ppm, based onthe weight of the amide respectively.

[0052] The invention will be illustrated in more detail by way of thefollowing examples, but the invention is not deemed to be limitedthereto.

COMPARATIVE EXAMPLES 1 TO 4 AND EXAMPLE 1

[0053] Catalyst for Hydration

[0054] A Raney copper alloy having a granular size of 80 mesh or lesswas developed with caustic soda (sodium hydroxide) by conventionalmethod, and then washed to prepare a Raney copper catalyst. During thepreparation and in subsequent handling, the contact of the catalyst witha gas containing oxygen such as air was avoided.

[0055] Catalytic Hydration Reaction

[0056] 400 g of the above-mentioned catalyst was placed in a SUSreactor, having a volume of about 2 liters equipped with a stirrer and acatalyst separator therein. Acrylonitrile and water from which dissolvedoxygen was previously removed by the use of a nitrogen gas were then fedat flow rates of 600 g/hr and 900 g/hr, respectively, and a reaction wascarried out at 120° C. The reaction solution was stirred together withthe catalyst to become a suspension, and this suspension was then passedthrough the catalyst separator and took out from the reactor as asubstantially catalyst-free solution. This reaction was continued for 3days.

[0057] Concentration

[0058] The obtained reaction solution was concentrated under reducedpressure by a batchwise technique so that the total amount of unreactedacrylonitrile and a part of unreacted water were distilled off, therebyobtaining an aqueous acrylamide solution having a concentration of about50% by weight. The thus obtained aqueous acrylamide solution containedcopper ions and acrylic acid.

[0059] In order to conduct a stability test, copper ions and acrylicacid contained in the solution were removed in the manner describedbelow.

[0060] Purification

[0061] A glass column was filled with 150 ml of a strongly acidic cationexchange resin Lewatit SP-112 (trade name, made by Bayer AG) which waspretreated with dilute hydrochloric acid in accordance with aconventional procedure.

[0062] Another glass column was filled with 300 ml of a weakly basicanion exchange resin Lewatit MP-64 (trade name, made by Bayer AG) whichwas pretreated with aqueous solution of caustic soda in accordance witha conventional procedure.

[0063] The aqueous acrylamide solution having a concentration of about50% by weight described above was then passed through these glasscolumns, firstly the column filled with the strongly acidic cationexchange resin and secondly the column filled with the weakly basicanion exchange resin, at a flow rate of 900 ml/hr.

[0064] In the obtained solution, the copper content was less than 0.01ppm, and the acrylic acid content was less than 1 ppm.

[0065] Thiourea, nitrobenzene, sodium acrylate and/or methoxyphenol wereadded to the 50% by weight acrylamide aqueous solution thus obtained, sothat each solution contains these additives in amounts shown in thetable 1. The pH of each solution is adjusted to 7.0 by adding causticsoda and sulfuric acid.

[0066] Preservation stability was evaluated by measuring a time perioduntil gelation occurred in each solution. For the measurement, a testpiece made of stainless steel was put into 100 ml of each solution, andthe solution was kept in a constant temperature bath of 40° C.

[0067] The results are shown in Table 1. TABLE 1 Sodium Methoxy-Preservation Thiourea Nitrobenzene Acrylate phenol stability (ppm/AAM)(ppm/AAM) (ppm/AAM) (ppm/AAM) (days) Comparative 0 0 0 20 14 Example 1Comparative 0 0 500 20 29 Example 2 Comparative 20 0 0 20 100 Example 3Comparative 0 20 500 20 70 Example 4 Example 1 20 0 500 20 130

[0068] According to the present invention, problems of polymerization ofan amide in its production process, during its deposition and during itspreservation are prevented. The invention provides markedly stable amidecontaining aqueous solution and crystalline amide.

What is claimed is:
 1. An amide composition comprising an amide whichhas an unsaturated bond, a sulfur-containing compound and a weak acidsalt.
 2. The amide composition of claim 1, wherein the sulfur-containingcompound is thiourea.
 3. The amide composition of claim 1, wherein theamide is an unsaturated aliphatic amide.
 4. The amide composition ofclaim 1, wherein the amide is acrylamide or methacrylamide.
 5. The amidecomposition of claim 1, wherein the sulfur-containing compound ispresent in an amount in the range of 1 to 100 ppm based on the weight ofthe amide.
 6. The amide composition of claim 1, wherein the weak acidsalt is a salt of an organic acid which has an acid dissociationexponent of 3.5 to 5.5.
 7. The amide composition of claim 1, wherein theweak acid salt is present in an amount in the range of 1 to 3,000 ppmbased on the weight of the amide.
 8. The amide composition of claim 1,wherein the sulfur-containing compound is thiourea, the weak acid saltis a salt of an organic acid having an acid dissociation exponent of 3.5to 5.5, the thiourea is present in an amount in the range of 1 to 100ppm based on the weight of the amide, and the salt of the organic acidis present in an amount in the range of 1 to 3000 ppm based on theweight of the amide.
 9. The amide composition of claim 1, wherein theamide is in a crystalline state.
 10. The amide composition of claim 1,wherein the composition is an aqueous solution.